1. Field of the Invention
The present invention relates to a polarization conversion dichroic mirror and a liquid crystal projector employing such a polarization conversion dichroic mirror.
2. Description of the Prior Art
FIG. 10 shows an example of a conventional liquid crystal projector. This liquid crystal projector is of the so-called separated-pupil illumination type, and is provided with a light source unit (U1), an illumination relay optical system (U2), a color separating/integrating cross dichroic prism (DP), reflection-type liquid crystal panels (PR, PG, and PB), a projection lens system (U3), and other components. The projection lens system (U3) includes a reflecting mirror (8) for directing illumination light (L1) to the reflection-type liquid crystal panels (PR, PG, and PB). The light source unit (U1) is composed of a light source (1), a reflector (2), a first lens array (3), a polarizing prism (4), a second lens array (5), a half-wave plate (6), a merging lens element (7), and other components.
The beam of randomly-polarized white light emitted from the light source (1) is first reflected from the reflector (2), is then separated into an S-polarized light component and a P-polarized light component by the polarizing-separating surface (4a) of the polarizing prism (4), and then forms a light source image on the second lens array (5). Then, the P-polarized light component is converted into an S-polarized light component by the half-wave plate (6). Thereafter, the light beam, now containing only an S-polarized light component, passes through the merging lens element (7). In this way, linearly-polarized white light is emitted as illumination light (L1) from the light source unit (U1). The illumination light (L1) is then relayed by the illumination relay optical system (U2) so as to be re-focused in the vicinity of the aperture stop (ST) (I: a virtual image plane).
Then, the illumination light (L1) is reflected from the reflecting mirror (8) disposed in the vicinity of the aperture stop (ST), and is thereby introduced into the projection lens system (U3). After passing through the rear portion of the projection lens system (U3), the illumination light (L1) enters the cross dichroic prism (DP). The cross dichroic prism (DP) has a B reflecting surface (RB) and an R reflecting surface (RR). Of the light components of three primary colors (R, G, and B) constituting the white light (L1), the B (blue) light component (BS) is reflected from the B reflecting surface (RB), and the R (red) light component (RS) is reflected from the R reflecting surface (RR). Thus, the cross dichroic prism (DP) transmits the G (green) light component (GS) straight and simultaneously reflects the R and B light components (RS and BS) in opposite directions. In this way, the illumination light (L1) is separated into three light components of different colors (BS, GS, and RS).
On the optical paths of the individual light components of three colors (BS, GS, and RS) thus separated are disposed the three reflection-type liquid crystal panels (PB, PG, and PR), which display the images of their respective light components. Thus, the individual light components of three colors (BS, GS, and RS) illuminate the display surfaces of their respective reflection-type liquid crystal panels (PB, PG, and PR), and are then reflected therefrom. The light components of three colors (BS, GS, and RS) reflected from the liquid crystal panels (PR, PG, and PB) are then integrated together by the cross dichroic prism (DP) so as to be formed into a projection light beam (L2), which is then projected through the projection lens system (U3) to form an enlarged, color-integrated image on a screen (not shown).
In the conventional liquid crystal projector described above, illumination light (L1) is made to contain only an S-polarized light component (BS, GS, and RS) before being directed into the cross dichroic prism (DP). For example, assume that the transmittance of the cross dichroic prism (DP) for S-polarized light drops to 50% at a cutoff wavelength of 580 nm on its R reflecting surface (RR) and at a cutoff wavelength of 510 nm on its B reflecting surface (RB). In this case, the G light component (GS) illuminating the liquid crystal panel (PG) covers a wavelength range from 510 nm to 580 nm, with its energy reduced to 50% at the wavelengths of 510 nm and 580 nm.
The G light component (GS) reflected from the liquid crystal panel (PG) is integrated into the projection light beam (L2), and is then transmitted through the R and B reflecting surfaces (RR and RB) of the cross dichroic prism (DP) once again. Here again, only 50% of the light having the wavelengths of 510 nm and 580 nm is transmitted through the R and B reflecting surfaces (RR and RB). Consequently, when the projection light beam (L2) reaches the screen, its energy at the wavelengths of 510 nm and 580 nm has dropped to as little as 25%. Thus, within the projection light beam (L2), the wavelength range covered by the G light component (GS) is narrowed to a wavelength range (for example, from 520 nm to 570 nm) in which a transmittance of approximately 70% (i.e. a two-way transmittance of 50%) or above is secured on the R and B reflecting surfaces (RR and RB).
The same description applies also to the R and B light components (RS and BS). The illumination light (L1) is separated into light components whose wavelength ranges are determined by the wavelength ranges (580 nm or above for the R light component, and 510 nm or below for the B light component) in which the R and G reflecting surfaces (RR and RG) offer a reflectance of 50% or more. In contrast, the projection light beam (L2) is composed of light components whose wavelength ranges are narrowed to the wavelength ranges (for example, 590 nm or above for the R light component, and 500 nm or below for the B light component) in which the R and G reflecting surfaces (RR and RG) offer a reflectance of 70% (i.e. a two-way reflectance of 50%) or above. These values are rough estimates made for a principal ray. For light rays traveling at different angles from principal rays, the wavelength ranges of the light components of three colors (BS, GS, and RS) are even narrower.
In other words, the conventional liquid crystal projector as shown in FIG. 10 suffers from a loss in light energy (i.e. a loss in the quantity of light) in the wavelength ranges around the boundary wavelengths between light components of different colors (i.e. around cutoff wavelengths). As shown in FIG. 11, the lost light becomes stray light (LG) that undergoes reflection and transmission over and over again in and around the cross dichroic prism (DP). Such stray light (LG) causes ghosts in the projected image and degrades the contrast thereof.